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We study $\mu - e$ conversion with sequential four generations. A large massfor the fourth generation neutrino can enhance the conversion rate by orders ofmagnitude. We compare constraints obtained from $\mu - e$ conversion usingexperimental bounds on various nuclei with those from $\mu \to e \gamma$ and$\mu \to e\bar e e$. We find that the current bound from $\mu - e$ conversionwith Au puts the most stringent constraint in this model. The relevant flavorchanging parameter $\lambda_{\mu e} = V^*_{\mu 4}V_{e4}^{}$ is constrained tobe less than $1.6\times 10^{-5}$ for the fourth generation neutrino mass largerthan 100 GeV. Implications for future $\mu -e$ conversion, $\mu \to e\gamma$and $\mu \to e\bar e e$ experiments are discussed.

$\mu - e$ Conversion With Four Generations